LABORATORY TWO Plate Tectonics and the Origin of Magma

Transcription

1 LABORATORY TWO Plate Tectonics and the Origin of Magma OBJECTIVES AND ACTIVITIES A. Infer whether expanding-earth or shrinking-earth hypotheses could explain plate tectonics and how mantle convection plays a role in causing plate tectonics. ACTIVITY 2.1: Is Plate Tectonics Caused by a Change in Earth s Size? (p , 43-44) ACTIVITY 2.2: Evaluate a Lava Lamp Model of Earth (p , 45-46) B. Understand how plate boundaries are identified and be able to measure and calculate some plate tectonic processes. ACTIVITY 2.3: Using Earthquakes to Identify Plate Boundaries (p. 35, 47-48) ACTIVITY 2.4: Analysis of Atlantic Seafloor Spreading (p. 35, 49-50) ACTIVITY 2.5: Plate Motions Along the San Andreas Fault (p. 35, 51-52) ACTIVITY 2.6: The Hawaiian Hot Spot and Pacific Plate Motion (p. 35, 53) ACTIVITY 2.7: Plate Tectonics of the Northwest United States (p. 35, 54) C. Use physical and graphical models of rock melting to infer how magma forms in relation to pressure, temperature, water, and plate tectonics. ACTIVITY 2.8: The Origin of Magma (p , 55-56) STUDENT MATERIALS (Remind students to bring items you check below.) laboratory manual laboratory notebook pencil with eraser metric ruler (cut from GeoTools sheet 1 or 2) calculator colored pencils (red and blue) visual estimation of percent chart (cut from GeoTools sheet 1 or 2) : : 14

2 INSTRUCTOR MATERIALS (Check off items you will need to provide.) ACTIVITY 2.1: Is Plate Tectonics Caused by a Change in Earth s Size? (p , 43-44) extra metric rulers (for students who forgot them but want to use one) ACTIVITY 2.2: Evaluate a Lava Lamp Model of Earth (p , 45-46) extra blue pencils (for students who forgot them) extra red pencils (for students who forgot them) lava lamp (turned on at least 1 hour ahead of time) and/or lava lamp video clip on IRC-DVD ACTIVITY 2.3: Using Earthquakes to Identify Plate Boundaries (p. 35, 47-48) extra metric rulers (for students who forgot them) extra red pencils or pens (for students who forgot them) ACTIVITY 2.4: Analysis of Atlantic Seafloor Spreading (p. 35, 49-50) extra metric rulers (for students who forgot them) extra blue pencils or pens (for students who forgot them) extra red pencils or pens (for students who forgot them) ACTIVITY 2.5: Plate Motions Along the San Andreas Fault (p. 35, 51-52) extra metric rulers (for students who forgot them) ACTIVITY 2.6: The Hawaiian Hot Spot and Pacific Plate Motion (p. 35, 53) extra metric rulers (for students who forgot them) ACTIVITY 2.7: Plate Tectonics of the Northwest United States (p. 35, 54) extra metric rulers (for students who forgot them) ACTIVITY 2.8: The Origin of Magma (p , 55-56) extra metric rulers (for students who forgot them) hot plate (one per group of students) sugar cubes (two per group of students) dropper with water or dropper bottle (one per group of students) aluminum foil (one sheet per group of students) or foil baking cups (two per group of students) crucible tongs (one per group of students) permanent felt-tip marker (one per group of students) 15

3 INSTRUCTOR NOTES AND REFERENCES 1. Refer to Laboratory 2 on the Internet site at for additional information and links related to all parts of this laboratory. 2. Metric and International System of Units (SI): refer to laboratory manual page x. 3. Mathematical conversions: refer to laboratory manual page xi. 4. To model Kinetic Theory, place small plastic or glass marbles in a clear plastic box or tray on an overhead projector. Hold the model still and elevated at one end, so the marbles form a close-packed array resembling a crystalline solid structure. Vibrate the model slightly to model a rise in kinetic energy and to start moving the marbles apart, as if melting is initiating. Vibrate the model more to model a greater rise in kinetic energy and to cause all of the marbles to move about independently, as if total melting or vaporization has occurred. (To model crystallization by decreasing kinetic energy, repeat these tasks in reverse.) Note: be sure to remind students that plumes of Earth s mantle are rock, not liquid magma or lava. 5. One lava lamp, or two, or three? Most lava lamps must be lighted for at least 1 hour before they exhibit active and obvious convection. It helps to have two or more lamps that have been turned on at different times, so the lava (wax) has varying amounts of kinetic energy. This helps students understand kinetic theory and how unequal amounts of heating affect the development and rate of convection. A lava lamp video clip is provided on the IRC-DVD. Note: be sure to remind students that Earth s mantle is rock, not liquid magma or lava. 6. Decompression melting. To help students visualize decompression melting, have them mix corn starch and water to make a corn starch suspension. Then have them try to roll some of the suspension into a ball and watch the suspension flow through their fingers. So long as the suspension is under pressure (i.e., while it is being rolled into a ball), it remains in a solid-like state. When the suspension is not under pressure (i.e., when a ball of suspension is placed on the palm of a hand), it flows in a liquid state. This is NOT decompression melting, but it helps students understand that pressure can prevent flowing even in materials that are normally fluid. 7. To model mantle plumes and hot spots, make a model of Earth s compositional layering first by placing clear corn syrup in a clear plastic cup (to represent Earth s mantle) and then by adding a few mm of water on top of it (to represent Earth s crust). To prepare material for a mantle plume, heat a small amount of corn syrup (with a few drops of red food coloring) on a hot plate. Fill a dropper with the hot red corn syrup, then squeeze some of it out onto the bottom of the plastic cup containing the cool syrup and water. Watch as the hot red corn syrup rises through the cooler corn syrup to form a long narrow plume and a pool of red syrup just beneath the water (crust). This is especially useful for having students entertain ideas about the origin of hot spots. 16

4 8. Flux melting. In the smelting industry, the term flux refers to materials such as fluxstone that are added to raw ore in order to lower the fusion (melting) temperature and produce slag. Experimental petrology has demonstrated that water lowers the fusion temperature of some minerals commonly found in granite, basalt, and peridotite; thereby initiating partial melting at lower, wet solidus temperatures. 9. Water in Earth's mantle. For information on recycling of water into Earth's mantle, refer to: C. Meade and R. Jeanloz Deep-focus earthquakes and recycling of water into Earth's mantle. Science 252: San Andreas fault slip rate. For a published estimate of the recurrence interval for very large earthquakes along the San Andreas fault north of San Francisco (221 +/- 40 yr) see: T.M. Niemi and N.T. Hall Late Holocene slip rate and recurrence of great earthquakes on the San Andreas fault in northern California. Geology 20: During the recurrence interval, the accumulated strain would be about 3.2 m (10 ft). However, Niemi and Hall (1992) also estimated that the Late Holocene rate of movement on the fault is about 2.4 cm/yr. For a published estimate of the recurrence interval for very large earthquakes along the San Andreas fault zone 70 km northeast of Los Angeles, California, see: T.E. Fumal, S.K. Pezzopane R.J. Weldon II, and D.P. Schwartz A 100-Year Average Recurrence Interval for the San Andreas Fault at Wrightwood, California. Science 259: They calculated a recurrence interval of about 100 years, a slip rate of about 2.5 cm per year, and a slip per very large earthquake of about 4 meters. ACTIVITY 2.1 ANSWERS AND EXPLANATIONS 2.1A. 17

5 2.1B. 2.1C. Students must be able to recognize divergent plate boundaries (red), transform plate boundaries (dashed, usually between segments of red divergent boundaries, but also including the San Andreas fault), and convergent boundaries (black with triangular teeth). Student estimations will vary, so it is good to have students share their estimates and obtain a class generalization. Most students correctly make the following visual estimation: 1. about 33% of Earth s plate boundaries are transform plate boundaries. 2. about 33% of Earth s plate boundaries are divergent plate boundaries. 3. about 33% of Earth s plate boundaries are convergent plate boundaries. 2.1D. Based on the answers to questions above, there is evidence for equal amounts of crustal compression, tension, and shear. Thus, it seems reasonable that Earth s size is not changing (i.e., Earth is staying about the same). 2.1E. As Earth's size does not seem to be changing (answer 2.1D), plate tectonics cannot be caused by a change in Earth's size. For Earth to remain the same size, it is moret likely that lithosphere is created at divergent boundaries, recycled back into the mantle at convergent boundaries, and neither created nor recycled at transform boundaries. ACTIVITY 2.2 ANSWERS AND EXPLANATIONS 2.2A. Students must observe a convecting lava lamp (that has been heating at least one hour) or a movie clip of a convecting lava lamp to answer these questions. 1. The lava moves from the base of the lamp to the top of the lamp, where it sits temporarily before sinking back to the bottom of the lamp. 2. Lava at the base of the lamp is heated by the light bulb. As the lava is heated, its kinetic energy level rises, which causes the lava to expand to a slightly greater volume and lower density. When the density of lava is less than the surrounding fluid, the lava rises. 3. Lava at the top of the lamp is cooling. As it cools, its kinetic energy level decreases, which causes the lava to contract into slightly less volume and higher density. When the density of lava is greater than the surrounding fluid, the lava sinks. 4. convection 18

6 2.2B. 1. Earth s mantle is like a lava lamp, because: mantle rocks are unequally heated like lava in the lava lamp. mantle rocks are heated at the base of the mantle, like lava in a lava lamp is heated at the base of the lava lamp. it has warmer rocks that rise like lava in a lava lamp. it has cooler rocks that sit atop the mantle or sink back into the mantle, like the masses of cooling lava at the top of the lava lamp. 2. Earth s mantle is different from a lava lamp, because: the mantle is rock, not lava or wax. the mantle is heated by Earth s outer core, but the lava lamp is heated by a light bulb. the mantle convects more slowly (i.e., cm/year) than the lava lamp (cm/second or cm/minute). 2.2C. By comparing lab manual Figures 2.4 and 2.3, students should observe that: 1. the warmer, less dense mantle rocks (red in Figure 2.4) mostly occur beneath divergent plate boundaries and hot spots. 2. the cooler, denser mantle rocks (blue in Figure 2.4) mostly occur beneath continents. 2.2D. The nature and detail of student cross sections will vary, but it should be at least a labeled, simple sketch like the one below. 19

7 ACTIVITY 2.3 ANSWERS AND EXPLANATIONS 2.3A. 2.3B 1. convergent 2. See black line (solid and dashed) on graph. 3. Lithospheric earthquakes occur above the line of the to surface of the subducting plate km 20

8 ACTIVITY 2.4 ANSWERS AND EXPLANATIONS 2.4A. 2.4B. Although points B and C were together 145 million years ago, they did not spread apart at exactly the same rate on opposite sides of the mid-ocean ridge. You can tell this because distance A-B is greater than distance A-C. 2.4C. How far apart are points B and C today? ~ 4000 km km 145 million years = 27.6 km/m.y. 2. There are 1000 m/km and 1000 mm/m, so there are 1,000,000 mm/km km 1,000,000 mm/km = 27,600,000 mm 27,600,000 mm 145,000,000 yr = 0.19 mm/yr 2.4D. When Africa and North America were together as part of one continent, points D and E were at the same location. They are now about 5400 to 5500 km apart (depending on how students measure the distance) km 27.6 km/m.y. = 195 m.y. AND 5500 km 27.6 km/m.y. = 199 m.y., so students should determine that Africa and North America were part of the same continent about m.y. ago (Early part of the Jurassic Period according to Figure 1.3 on page 4 of the Lab Manual). 2.4E = 235 yr AND 235 yr 0.19 mm/yr = 44.6 mm So, 44.6 mm m/mm = m Students should calculate that Africa and North America have moved apart by only a small fraction of one meter since the United States formed in

9 ACTIVITY 2.5 ANSWERS AND EXPLANATIONS 2.5A. 1. If the rock body formed 25 million years ago, and faulting displaced parts of it from 320 km (the minimum separation in Figure 1.10) to 380 km (the maximum separation in Figure 1.10), then the rate of motion is from 320 km /25 m.y. to 380 km/25 m.y., or 32,000,000 cm/25,000,000 yr to 38,000,000 cm./25,000,000 yr, or 32 cm/25 yr to 38 cm/25 yr, which equals: 1.2 to 1.5 cm/yr. 2. Students have already found that offset along the fault is about km, or 320, ,000 meters. So 320, ,000 meters 5 meters/offset equals 64,000 76,000 earthquakes with 5 meter offsets in the past 25 million years. 64,000 to 76,000 earthquakes/25,000,000 years = 64 to 76 earthquakes/25,000 yr, or one 5m earthquake every 390 to 329 years. 2.5B. 1. The North American Plate (north of the red San Andreas Fault) is moving about 20 mm/yr. The Pacific Plate (south of the red San Andreas Fault) is moving about 40 mm/yr. So, the Pacific Plate is moving about twice (2 times) as fast as the Pacific Plate here. 2. Note half arrows on map (showing the relative motion of the fault). 22

10 ACTIVITY 2.6 ANSWERS AND EXPLANATIONS 2.6A. 1. From million years ago the plate moved about 300 km west-northwest. This is a rate of: 30,000,000 cm 3,100,000 yr = 9.7 cm/yr west-northwest 2. From 1.6 million years ago to now, the plate has moved about 255 km almost due northwest (as measured from the easternmost volcano on Molokai to Kilauea volcano on Hawaii). This is a rate of: 25,500,000 cm 1,600,000 yr = 15.9 cm/yr due northwest 3. The Pacific Plate has moved about 1.6 times faster (and in a more northerly direction) over the past 1.6 million years than it moved from million years ago. 4. The Emperor Seamount chain and the Hawaiian Island chain seem to be one long chain of volcanic islands. The Emperor Seamount chain seems to be an older chain of islands that formed like the Hawaiian Island chain did from volcanic activity beneath the Pacific Plate as the plate moved over the Hawaiian Hot Spot in the mantle. Note: You can simulate how a hot spot burns a line of volcanoes into a plate moving over it. Remove the cover from a very large (poster size) permanent black felt-tip pen and place it tip-up on a table. Hold the pen while you slowly slide a piece of white paper over the pen tip. The ink from the pen will bleed through the paper. If you do this with a very slow motion and stop periodically, then you will create a pattern of islands separated by lines. 5. From 60 million to 40 million years ago, the Pacific Plate moved almost due north over the Hawaiian Hot Spot. Since 40 Ma, the Pacific Plate has moved nearly due northwest over the Hawaiian Hot Spot (with minor variations as noted in Question 15c above). ACTIVITY 2.7 ANSWERS AND EXPLANATIONS 2.7A. 1. The circular deformation zones that formed over the Yellowstone Hot Spot have ages ranging from 13.8 million years old in the southeast to 0.5 million years old in the northeast part of the chain. Therefore, the oldest deformation zone must have formed and moved southwest away from the hot spot. The North American Plate is, therefore, moving southwest over the Yellowstone Hot Spot. 2. The North American Plate has moved about km over the past 11 million years, so its average rate of motion has been: 35,000,000 to 40,000,000 cm 11,000,000 yr = 3.2 to 3.6 cm/yr 23

11 2.7B 1. The plate east of the ridge has moved about 230 km over the past 8 million years, so the average rate of seafloor spreading east of the ridge has been 23,000,000 cm 8,000,000 yr = 2.9 cm/yr. 2. The seafloor rocks that should be present along line segment C-D must have subducted beneath the North American Plate as it moved westward (southwestward). 3. The plate boundary along the red line in Figure 2.12 must be a convergent plate boundary with a subduction zone. 4. As seafloor subducts along line segment C-D it undergoes heating along the geothermal gradient. The water acts as a flux to superjacent rocks of the mantle wedge and eventually causes them to undergo hydration and partial melting (flux melting). The resulting magma rises to form the Cascade Range of volcanic mountains. ACTIVITY 2.8 ANSWERS AND EXPLANATIONS 2.8A. 1. about 750 C 2. about 1000 C 3. solid (It is left of the solidus in the field labeled 100% solid peridotite rock.) 4. It would partially melt. If you move point X to the right until it is below the temperature of 1750 C, then it is located in the field of partial melting, between the solidus and liquidus. 5. It would melt completely because it would be located in the field to the right of the liquidus, which is labeled 100% liquid magma. 2.8B. 1. about 40 km and 13,000 atm 2. decompression melting 3. Decompression melting could occur where a plume of hot mantle peridotite (red in Figure 2.6) rises to a shallower depth and lower pressure where melting can occur. This may be happening along divergent plate boundaries (like ocean ridges and rifts) and at hot spots. 2.8C. To begin partial melting, the peridotite at point X in Figure 2.7 must be uplifted to a depth of about 40 km (and a pressure of about 13,000 atm) or else it must be heated to about 1450 C. 2.8D. 1. The wet sugar cube melted first. 2. water 3. The solidus, liquidus, and all fields would move to the left (to lower temperatures). 4. subduction zones 24

12 2.8E. 1. divergent plate boundary 2. decompression melting 3. A mass/plume of hot mantle peridotite rises close to Earth s surface, where it encounters lower pressure and melts to form basaltic magma. The magma erupts along the oceanic ridge, where it pushes the existing rock plate apart. This pushes the plates and starts the process of seafloor spreading. 2.8F. 1. convergent plate boundary 2. flux melting 3. Wet seafloor basalt subducts beneath the less dense continental edge of an adjacent plate. The basalt dehydrates and hydrates the base of the continental crust. Flux melting causes formation of magma, which rises to form a line of volcanoes (volcanic arc). 25